Shares in CIA jumped 8p to 337p on news of the link, which will increase the billings of the media group by about a sixth.
CIA is to take over the running of CDP's media buying, planning and strategy arm, the side that decides where advertising will be placed among television stations, radio, newspapers and the like and then buys the airtime or advertising space.
CDP's media operations have billings of about pounds 50m a year, compared with a prediction by CIA's brokers, Kleinwort Benson, that the media group will have billings of about pounds 300m this year.
Peter Toynton, CIA's finance director, said the joint venture was the largest of its kind concluded by CIA. 'They will provide the clients and we will provide the research and expertise,' he said.
The venture is expected to lead to further links between the two groups, and the possibility of other ventures, perhaps in Europe, is also being explored.
The stock market was also excited by the involvement of Dentsu, which has consistently maintained its position as one of the largest marketing services groups in the world over recent years and currently ranks as the world's largest single advertising agency.
However, Mr Toynton was cautious about any links with the Japanese group.
'We would be very happy to have links with Dentsu but there is nothing on the cards at the moment,' he said. 'But who knows where things may lead.'
Mr Toynton said CIA was ambitious to grow, using the cash it has been able to accumulate despite one of the worst recessions to have hit the advertising industry.Reuse content